Photosensitive resin printing plates general as plate materials for flexographic printing are formed as follows. First, a solid or liquid photosensitive resin as a raw material is irradiated with active light, and only photosensitive layers of relief portions are cured by a radical polymerization reaction (exposure process). Then, uncured resin except the relief portions is dissolved and removed with a predetermined wash out solution (developing solution), or swollen and dispersed and mechanically removed (development process). This forming method is one for making cured portions only emerging on the plate surface as a relief. The forming method is preferably used because of being capable of forming a fine relief in a short time.
Among them, the method of using a liquid photosensitive resin takes only a short time required for producing printing plates and can cope with short delivery times. The method can also reduce production costs because of being capable of recovering and reutilizing uncured resin. Using the method further gives a favorable flexibility to the surface of printing plates formed. From these points, the method using a liquid photosensitive resin widely spreads.
In the case of printing on the surface of corrugated board sheets, printing plates formed using a liquid photosensitive resin, that is, liquid photosensitive resin printing plates, are preferably used. Corrugated board sheets typically have a structure in which liners (paper boards) are laminated on a corrugating medium, or a structure laminated from the former structures. The printing surface of the sheets has a corrugating shape due to the presence of the corrugating medium, and the sheets themselves are flexible due to the hollow structure. Hence, liquid photosensitive resin printing plates having a favorable flexibility are preferably used.
A producing process of corrugated boards distributed as products commonly contains a sheet producing step, a printing step, a die cutting step, a box making step, and the like. Major corrugated board manufacturers performing integrally these steps have a problem caused by paper pieces and paper dusts in some cases. In the sheet producing step in the corrugated board producing process, paper pieces and paper dusts are liable to be generated. Adhering of the generated paper pieces and paper dusts to plate surfaces of printing plates in the printing step causes printing defects (spoilage). In order to remove the paper pieces and paper dusts adhered to plate surfaces, printing must be suspended. These problems lead to an increase in working hours, a decrease in productivity, and an increase in production costs.
Moreover, since the mix rate of recycled raw materials such as newspaper and used paper for base papers used for corrugated boards has recently increased year by year, the amount of paper pieces and paper dusts generated in production sites increases. The printing quality demanded for corrugated board cases to package beverages, electric household appliances and the like has recently been raised increasingly. Under such a situation, a liquid photosensitive resin printing plate capable of reducing the influence of problems caused by paper pieces and paper dusts is demanded in corrugated board printing.
In the case of obtaining a flexible printing plate demanded for corrugated board printing from a liquid photosensitive resin, relief portions insolubilized to a developing solution by a photoradical reaction often leave portions remaining such as those having a relatively low polymerization degree, those having a high residual ratio of unreacted monomers and those having a loose crosslinking structure. Such portions bring tackiness on relief surfaces. Again irradiating a relief after development having such portions with active light (post-exposure) enables to reduce the tackiness.
Patent Document 1 describes a liquid photosensitive resin composition in which a polyurethane (meth)acrylate prepolymer based on an amine-modified polyether is blended with benzophenone. The patent document describes a technology in which such a liquid photosensitive resin composition is subjected to an exposure and development treatment, and thereafter, is irradiated with germicidal radiation to make relief surfaces nontacky.
Patent Document 2 describes a liquid photosensitive resin composition composed of a polyurethane prepolymer containing a specific benzoic acid derivative as a tackiness-reducing additive. The patent document describes a technology in which such a liquid photosensitive resin composition is subjected to an exposure and development treatment, and thereafter, is irradiated with germicidal radiation to make relief surfaces nontacky.
Patent Document 3 proposes a method of making nontacky surfaces of many flexographic printing plates obtained by a producing method containing a wash out (development) step and a post-exposure step. The method involves using a wash out solution containing a hydrogen abstracting agent (for example, benzophenone). The patent document describes a technology in which uncured resin is washed away using such a wash out solution, and thereafter, the surfaces of flexographic printing plate reliefs are irradiated with a radiation to activate the hydrogen abstracting agent, for example, germicidal light or the like for benzophenone, to make the surfaces nontacky.
Meanwhile, the relief depth of printing plates used especially in corrugated board printing often reaches 2 mm or more. Such a relief is liable to break, that is, to cause plate chipping.
Under such a situation, Patent Document 4 describes a flexographic printing plate achieving a notch cracking resistance of 20 sec. or more. The notch cracking resistance is an index of the durability required in corrugated board printing. Such a flexographic printing plate is formed of a specific liquid photosensitive resin composition containing a urethane prepolymer composed of a mixture of a polyether polyol and a polyester polyol, a monofunctional monomer, and a polyfunctional monomer.    Patent Document 1: Japanese Patent Application Laid-Open No. 08-169925    Patent Document 2: Japanese Patent Application Laid-Open No. 2000-206677    Patent Document 3: Japanese Patent Application Laid-Open No. 09-288356    Patent Document 4: Japanese Patent Application Laid-Open No. 04-95959